1. Technical Field
The present disclosure relates to electrosurgery. More particularly, the present disclosure relates to electrosurgical generators and methods that use a multi-stage power converter for generating electrosurgical energy.
2. Background of Related Art
Electrosurgery involves the application of high-frequency electric current to cut or modify biological tissue during an electrosurgical procedure. Electrosurgery is performed using an electrosurgical generator, an active electrode, and a return electrode. The electrosurgical generator (also referred to as a power supply or waveform generator) generates an alternating current (AC), which is applied to a patient's tissue through the active electrode and is returned to the electrosurgical generator through the return electrode. The alternating current typically has a frequency above 100 kilohertz (kHz) to avoid muscle and/or nerve stimulation.
During electrosurgery, the AC generated by the electrosurgical generator is conducted through tissue disposed between the active and return electrodes. The tissue's impedance converts the electrical energy (also referred to as electrosurgical energy) associated with the AC into heat, which causes the tissue temperature to rise. The electrosurgical generator controls the heating of the tissue by controlling the electric power (i.e., electrical energy per time) provided to the tissue. Although many other variables affect the total heating of the tissue, increased current density usually leads to increased heating. The electrosurgical energy is typically used for cutting, dissecting, ablating, coagulating, and/or sealing tissue.
The two basic types of electrosurgery employed are monopolar and bipolar electrosurgery. Both of these types of electrosurgery use an active electrode and a return electrode. In bipolar electrosurgery, the surgical instrument includes an active electrode and a return electrode on the same instrument or in very close proximity to one another, which cause current to flow through a small amount of tissue. In monopolar electrosurgery, the return electrode is located elsewhere on the patient's body and is typically not a part of the electrosurgical instrument itself. In monopolar electrosurgery, the return electrode is part of a device typically referred to as a return pad.
As shown in FIG. 4, the ideal output power characteristic of an electrosurgical (ES) generator is constant current 410, followed by constant power 420, which is, in turn, followed by constant voltage 430 as a function of the increasing output impedance. To achieve this output power characteristic, the ES generator executes a control loop that samples the output voltage and current, calculates power and/or impedance based on the sampled output voltage and current, feeds the calculated power and/or impedance through a digital compensator, and then adjusts a control signal (e.g., adjusts the pulse width or phase shift of the control signal) that controls the power stage. This control loop, however, may be relatively slow compared to the switching frequency of the power stage. Therefore, the power stage may under-deliver or over-deliver the desired power to the tissue until the control loop catches up with the power stage and the system reaches steady state. For this and other reasons, current ES generators may fall short of providing the ideal output power characteristic.